In the aeronautical industry, the current trend in turbofan design is to increase the bypass ratio in order to improve fuel consumption through increased propulsive efficiency and to reduce perceived noise on ground. In conventional turbofan engines, the pursuit of higher propulsive efficiency requires increasing the fan diameter in order to produce a higher by-pass mass flow at lower slipstream velocities. The incremental evolution of the current state of the art envisages Very High Bypass Ratio (VHBPR) turbofans (BPR greater than 15) having fan diameters of up to 174″.
As the continuous increase of the fan diameter generates problems of ground clearance and engine integration and installation in the wing or fuselage, an innovative turbofan engine concept has been proposed consisting in splitting the secondary by-pass flow into a plurality of fans of smaller diameter, all connected to the power generation unit (known as multi-fan engines).
For example U.S. Pat. No. 6,792,745 B2 discloses a turbofan jet engine having a housing and an engine core disposed in the housing. The engine core includes at least a compressor, turbine, and a drive shaft. The drive shaft defines a drive shaft axis. A plurality of fans is disposed in the housing and each fan rotates about a separate fan axis. Each of the fan axes is axially offset from the drive shaft axis. The turbofan jet engine further includes a drive system operably interconnecting the engine core and fans so as to drive the rotation of the fans and selectively disengage selected fans from the engine core.
EP 1,916,406 A2, U.S. Pat. No. 8,015,796 B2, U.S. Pat. No. 8,402,740 B2 and US 2013/0223991 A1 also disclose multi-fan engines.
In the above-mentioned prior art, each fan is driven by a shaft connected by a particular mechanical arrangement to the shaft driven by the turbine of the engine core. In the case of U.S. Pat. No. 6,792,745 said mechanical arrangement is a gearbox drive system operably coupled between the turbine-driven shaft and the shafts of the fans.
The mechanical arrangements described in the above-mentioned prior art raise a number of problems, particularly related to their capability to transmit power between fans through a mechanical gear-box of acceptable size and weight, so that new approaches to multi-fan engines are highly sought after by the aeronautical industry.